Switched-mode power supply device

ABSTRACT

An error amplifier outputs an error voltage to the primary side as a feedback signal indicating a state of the load. An oscillator controls a switching frequency of the switching operation such that the switching frequency is reduced from a highest normal frequency to a lowest normal frequency based on the feedback signal in a load range from a heavily-loaded state to a lightly-loaded state in a normal oscillating operation that continuously performs the switching operation. An intermittent oscillation control circuit intermittently performs the switching operation by stopping the switching operation based on the feedback signal when the load is lighter than a previously set reference. The oscillator increases the switching frequency from an intermittent-operation frequency lower than the lowest normal frequency in an intermittent oscillation duration in which the switching operation is performed in the intermittent oscillating operation.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority based on 35 USC 119 from prior JapanesePatent Application No. 2014-251769 filed on Dec. 12, 2014, entitled“SWITCHED-MODE POWER SUPPLY DEVICE”, the entire contents of which arehereby incorporated by reference.

BACKGROUND

The disclosure relates to a switched-mode power supply device configuredto control an output voltage through a switching operation.

A switched-mode power supply device performs an intermittent oscillatingoperation that intermittently performs a switching operation in alightly-loaded state (see Japanese Patent Laid-open No. 2007-215316:Patent literature 1, for example). The switched-mode power supply deviceaccording to Japanese Patent Laid-open No. 2007-215316 is provided withan upper limit frequency in accordance with a feedback voltage andconfigured to perform an intermittent oscillating operation that stopsafter being turned on a previously set number N of times.

SUMMARY

An embodiment provides a switched-mode power supply device thatcomprises a transformer comprising a primary winding in a primary sideto which a DC voltage is applied and a secondary winding, a switchingcircuit connected to the primary winding, a rectifying-smootheningcircuit that rectifies and smoothens a pulse voltage generated at thesecondary winding, by switching the switching element, an output unitthat outputs an output voltage obtained by the rectifying-smootheningcircuit to a load, an error amplifier that outputs an error voltage tothe primary side as a feedback signal indicating a state of the load,the error voltage being obtained by comparing the output voltage to areference voltage, an oscillator that controls a switching frequency ofthe switching operation such that the switching frequency is reducedfrom a highest normal frequency to a lowest normal frequency based onthe feedback signal in a load range from a heavily-loaded state to alightly-loaded state in a normal oscillating operation that continuouslyperforms the switching operation, and an intermittent oscillationcontrol circuit that intermittently performs the switching operation bystopping the switching operation based on the feedback signal when theload is lighter than a previously set reference, wherein the oscillatorincreases the switching frequency from an intermittent-operationfrequency lower than the lowest normal frequency in an intermittentoscillation duration in which the switching operation is performed inthe intermittent oscillating operation.

BRIEF DESCRIPTION OF DRAWING

FIG. 1 is a circuit configuration diagram of the configuration of aswitched-mode power supply device according to an embodiment;

FIG. 2 illustrates a circuit configuration of a controller ICillustrated in FIG. 1 according to a first embodiment;

FIG. 3 is a graph for describing an oscillation frequency changeoperation of an OSC illustrated in FIG. 2;

FIG. 4 shows graphs for describing an oscillation frequency changeoperation in an intermittent oscillating operation (a lightly-loadedstate with a relatively large load) of the OSC illustrated in FIG. 2;

FIG. 5 shows graphs for describing an oscillation frequency changeoperation of the OSC illustrated in FIG. 2 in an intermittentoscillating operation (at microcomputer standby);

FIG. 6 shows graphs for describing an oscillation frequency changeoperation of the OSC illustrated in FIG. 2 in an intermittentoscillating operation (at several W control);

FIG. 7 illustrates a circuit configuration of the controller ICillustrated in FIG. 1 according to a second embodiment; and

FIG. 8 illustrates a circuit configuration of the controller ICillustrated in FIG. 1 according to a third embodiment.

DETAILED DESCRIPTION

As illustrated in FIG. 1, a switched-mode power supply device accordingto an embodiment includes rectifying circuit DB, smoothing capacitorsC1, C2, and C3, transformer T, controller IC 1, rectifying diodes D1 andD2, error amplifier (E/A) 2, light-emitting diode PC1 andlight-receiving transistor PC2 included in a photo coupler,current-detection register Rocp, registers R1, R2, and R3, and capacitorC4.

Alternating-current input terminals ACin1 and ACin2 of rectifyingcircuit DB including bridge diodes are connected tocommercially-available alternating-current source AC, an input voltagefrom which is provided with full-wave rectification to be output fromrectifying circuit DB. Smoothing capacitor C1 connects arectified-output positive terminal and a rectified-output negativeterminal of rectifying circuit DB. The rectified-output negativeterminal of rectifying circuit DB is connected to an earth terminal.This configuration enables the input voltage from commercially-availablealternating-current source AC to be rectified and smoothed throughrectifying circuit DB and smoothing capacitor C1, whereby adirect-current voltage is obtained.

Controller IC 1 includes a built-in switching element such as a powermetal oxide semiconductor field effect transistor (MOSFET), and abuilt-in control circuit to control switching of the switching element.Controller IC 1 includes a D/ST (MOSFET drain; starting current input)terminal, an S/OCP (MOSFET source; overcurrent protection) terminal, aVcc (control circuit power-supply voltage input) terminal, a FB/OLP(feedback signal input; overload protection signal input) terminal, anda GND terminal.

Transformer T includes primary winding P, auxiliary winding D, andsecondary winding S, and supplies electric power from a primary side(input side) to a secondary side (load side). The rectified-outputpositive terminal of rectifying circuit DB is connected to one end ofprimary winding P of transformer T, whereas the other end of primarywinding P of transformer T is connected to the D/ST terminal ofcontroller IC 1. The S/OCP terminal of controller IC 1 is connected toan earth terminal through register Rocp. With this configuration,turning on and off of the built-in switching element of controller IC 1is controlled to transfer electric power provided to primary winding Pof transformer T to secondary winding S of transformer T, therebygenerating a pulsating flow through secondary winding S of transformerT. Current-detection register Rocp is a register that is connected tothe S/OCP terminal and detects, as voltage signal V_(ocp), currentflowing through the built-in switching element of controller IC 1.Controller IC 1 has an overcurrent protection (OCP) function to restrictelectric power supplied to the secondary side when voltage signalV_(ocp) corresponding to the current flowing through the switchingelement is equal to or larger than a previously set overcurrentthreshold.

Both ends of secondary winding S of transformer T are connected throughsmoothing capacitor C2 via rectifying diode D1. Rectifying diode D1 andsmoothing capacitor C2 serve as a secondary-side rectifying-smoothingcircuit. Voltage induced across secondary winding S of transformer T isrectified and smoothed through rectifying diode D1 and smoothingcapacitor C2, and voltage between terminals of smoothing capacitor C2 isoutput from an output terminal as output voltage Vo. A line connected tothe positive terminal of smoothing capacitor C2 is a power-supply line,whereas a line connected to the negative terminal of smoothing capacitorC2 is a GND line connected to the earth terminal.

Error amplifier 2 connects the power-supply line and the GND line inseries. Error amplifier 2 connecting the power-supply line and the GNDline compares output voltage Vo to a reference voltage so as to controlcurrent flowing through light-emitting diode PC1 of the photo coupler inaccordance with an error voltage between output voltage Vo and thereference voltage. The FB/OLP terminal of controller IC 1 is connectedto the earth terminal through light-emitting diode PC2 and capacitor C4connected in parallel. This configuration enables a feedback (FB) signalin accordance with the error voltage between output voltage Vo and thereference voltage to be transmitted from light-emitting diode PC1 on thesecondary side to light-receiving transistor PC2 on the primary side andinput as FB voltage V_(FB) to the FB/OLP terminal of controller IC 1.Controller IC 1 controls a duty ratio of the switching element based onFB voltage V_(FB) input to the FB/OLP terminal, and thus the amount ofelectric power supplied to the secondary side.

Smoothing capacitor C3 connects both ends of auxiliary winding D oftransformer T through register R3 and rectifying diode D2, and aconnection point of rectifying diode D2 and smoothing capacitor C3 isconnected to the Vcc terminal of controller IC 1. This configurationenables voltage generated across auxiliary winding D to be rectified andsmoothed through rectifying diode D2 and smoothing capacitor C3 andsupplied as IC power-supply voltage Vcc to the Vcc terminal ofcontroller IC 1.

First Embodiment

The following describes a circuit configuration of controller IC 1illustrated in FIG. 1 according to a first embodiment with reference toFIG. 2.

As illustrated in FIG. 2, controller IC 1 includes switching element Q1including an N-channel power MOSFET, drive circuit 11, OSC (internaloscillator) 12, regulator 13, timer circuit 14, variable voltage V_(R),overcurrent threshold voltage V_(th1), burst threshold voltage Vth₂, ORcircuits OR1 and OR2, flip-flop FF1, comparators COMP1, COMP2, COMP3,and COMP4, and register R4.

The drain terminal of switching element Q1 is connected to the D/STterminal, the source terminal thereof is connected to the S/OCPterminal, and the gate terminal thereof is connected to drive circuit 11that controls turning on and off of switching element Q1 by outputtingdrive signal DRV for driving switching element Q1. Drive circuit 11receives an inverted output from OR circuit OR1.

The input terminal of OR circuit OR1 receives an output from OSC 12 andan output from inverted output terminal Q- of flip-flop FF1. The Sterminal of flip-flop FF1 is connected to the output of OSC 12, whereasthe R terminal of flip-flop FF1 is connected to the output of OR circuitOR2. Flip-flop FF1 serves as a PWM latch circuit. With thisconfiguration, when a clock signal output from OSC 12 is at an L leveland flip-flop FF1 is set with an output signal from inverted outputterminal Q- being at the L level, an output signal at an H level from ORcircuit OR1 is input to drive circuit 11, so that switching element Q1is turned on. In this manner, the clock signal output from OSC 12controls timing of turning on a switching operation of switching elementQ1, and the switching frequency of the switching operation is set to bethe oscillation frequency of the clock signal output from OSC 12.

The S/OCP terminal is connected to the non-inverting terminal ofcomparator COMP1 and the non-inverting terminal of comparator COMP2.Comparator COMP1 is an OCP comparator that detects overcurrent. Theinverting terminal of comparator COMP1 is connected to overcurrentthreshold voltage V_(th1). When voltage signal V_(ocp) from the S/OCPterminal corresponding to drain current ID flowing through switchingelement Q1 is equal to or higher than overcurrent threshold voltageV_(th1), comparator COMP1 outputs an output signal at the H level. ThisH-level output signal of comparator COMP1 resets flip-flop FF1 throughOR circuit OR2, and sets the output signal of OR circuit OR1 to the Llevel, so that drive circuit 11 turns off switching element Q1accordingly.

Comparator COMP2 is a current sense converter for performing a feedbackcontrol of the duty ratio of switching element Q1 based on a FB signalinput as FB voltage V_(FB) to the FB/OLP terminal. The inverting inputterminal of comparator COMP2 is connected to the FB/OLP terminalconnected to reference voltage Reg through register R4. Comparator COMP2compares voltage signal V_(ocp) and FB voltage V_(FB), and outputs anoutput signal at the H level when voltage signal V_(ocp) is equal to orhigher than FB voltage V_(FB). This H-level output signal of comparatorCOMP2 resets flip-flop FF1 through OR circuit OR2, and sets an outputsignal of OR circuit OR1 to the L level, so that drive circuit 11 turnsoff switching element Q1 accordingly. In this manner, the duty ratio ofthe output signal of OR circuit OR1 is controlled based on the FB signalas a PWM signal.

Comparator COMP3 is a burst comparator that controls an intermittentoscillation of switching element Q1 based on a FB signal. In otherwords, comparator COMP3 serves as an intermittent oscillation controlcircuit configured to control the intermittent oscillation thatintermittently performs the switching operation by stopping theswitching operation when it is determined based on the FB signal that aload is lighter than a previously set reference. The non-inverting inputterminal of comparator COMP3 is connected to burst threshold voltageVth₂. When FB voltage V_(FB) is lower than burst threshold voltage Vth₂,comparator COMP3 outputs an output signal at the H level. This H-leveloutput signal of comparator COMP3 resets flip-flop FF1 through ORcircuit OR2, and sets an output signal of OR circuit OR1 to the L level,so that drive circuit 11 turns off switching element Q1 accordingly.Switching element Q1 is kept turned off until FB voltage V_(FB) becomesequal to or larger than burst threshold voltage Vth₂. When FB voltageV_(FB) becomes equal to or higher than burst threshold voltage Vth₂, theoutput signal of comparator COMP3 is set to the L level, and theswitching operation of switching element Q1 is started. Thus, (ON/OFF)timing of an intermittent oscillating operation is determined byfluctuation of FB voltage V_(FB), and the intermittent oscillatingoperation of switching element Q1 is performed in a load range from alightly-loaded state to an unloaded state. Since the (ON/OFF) timing ofthe intermittent oscillating operation is determined by variation of FBvoltage V_(FB), a switching operation time of switching element Q1depends on a load in the intermittent oscillating operation.

Comparator COMP4 is a comparison circuit that compares IC power-supplyvoltage Vcc of the Vcc terminal and variable voltage V_(R). Thenon-inverting input terminal of comparator COMP4 is connected to the Vccterminal, whereas the inverting input terminal thereof is connected tovariable voltage V_(R). Variable voltage V_(R) is set to first referencevoltage Von (for example, 15V) when comparator COMP4 outputs an outputsignal at the L level, and set to second reference voltage Voff (forexample, 10V) lower than first reference voltage Von when comparatorCOMP4 serving as an UVLO circuit outputs an output signal at the Hlevel. With this configuration, the output signal of comparator COMP4has a hysteresis characteristic. Smoothing capacitor C3 illustrated inFIG. 1 is charged by a starter circuit not illustrated. Smoothingcapacitor C3 is at the H level when IC power-supply voltage Vcc islarger than first reference voltage Von, and at the L level when ICpower-supply voltage Vcc is equal to or lower than second referencevoltage Voff.

The output terminal of comparator COMP4 is connected to regulator 13.Regulator 13 is supplied electric power from the Vcc terminal, and whenthe output signal of comparator COMP4 is at the H level, operates tosupply power-supply voltage for operating each component of controllerIC 1. Specifically, the output signal of comparator COMP4 is a signalfor controlling on and off of controller IC 1, and is at the H level ina steady operation of controller IC 1 (when the switching operation ison) while comparator COMP4 is serving as an UVLO circuit. Accordingly,first reference voltage Von set to variable voltage V_(R) is a voltageto start the operation of controller IC 1, and second reference voltageVoff set to variable voltage V_(R) is a voltage to stop the operation ofcontroller IC 1.

When the output signal of comparator COMP3 is turned off by shiftingfrom the H level to the L level, timer circuit 14 outputs anoscillation-start detection signal to OSC 12 and starts measurement ofoscillation duration (hereinafter, referred to as an intermittentoscillation duration) in the intermittent oscillating operation. Then,timer circuit 14 outputs a first frequency switching signal to OSC 12when previously set time T1 has passed since the outputting of theoscillation-start detection signal, and outputs second frequencyswitching signal to OSC 12 when previously set time T2 has passed sincethe outputting of the first frequency switching signal. Time T1 and timeT2 may be individually set as appropriate, and may be set equal to eachother.

OSC 12 has a frequency change function to change the oscillationfrequency of a clock signal to be output. OSC 12 reduces the oscillationfrequency based on a FB signal in a load range from a heavily-loadedstate to a lightly-loaded state and reduces the switching frequency ofthe switching operation in a normal oscillating operation thatcontinuously performs the switching operation. As illustrated in FIG. 3,when FB voltage V_(FB) is equal to or higher than frequency-reductionstart voltage Va, OSC 12 sets the oscillation frequency to a highestnormal frequency. When FB voltage V_(FB) is in a range offrequency-reduction start voltage Va to frequency-reduction end voltageVb lower than frequency-reduction start voltage Va, OSC 12 reduces theoscillation frequency from the highest normal frequency to a lowestnormal frequency in accordance with FB voltage V_(FB). When FB voltageV_(FB) is equal to or lower than frequency-reduction end voltage Vb, OSC12 sets the oscillation frequency to the lowest normal frequency. FIG. 3illustrates an example in which the oscillation frequency is reducedproportionally to FB voltage V_(FB), but the oscillation frequency maybe reduced in stages.

FIG. 4 shows graphs indicating FB voltage V_(FB), drain current ID ofswitching element Q1, and the oscillation frequency of OSC 12 in theintermittent oscillating operation in the lightly-loaded state with arelatively large load. In FIG. 4, a time axis is illustrated longer asit goes down. As illustrated in FIGS. 4A to 4D, when FB voltage V_(FB)becomes equal to or higher than burst threshold voltage Vth₂ at time t0,the output signal of comparator COMP3 becomes the L level, timer circuit14 inputs the oscillation-start detection signal to OSC 12, and OSC 12sets the oscillation frequency to a first intermittent-operationfrequency lower than the lowest normal frequency. This starts theswitching operation at first intermittent-operation frequency switchingelement Q1.

Timer circuit 14 outputs the first frequency switching signal to OSC 12at time t1 at which time T1 have passed since the outputting of theoscillation-start detection signal. Having received the first frequencyswitching signal, OSC 12 switches the oscillation frequency to a secondintermittent-operation frequency higher than the firstintermittent-operation frequency and lower than the lowest normalfrequency. This switches the switching operation of switching element Q1to the second intermittent-operation frequency.

Timer circuit 14 outputs the second frequency switching signal to OSC 12at time t2 at which time T2 have passed since the outputting of thefirst frequency switching signal. Having received the second frequencyswitching signal, OSC 12 switches the oscillation frequency to thelowest normal frequency. This switches the switching operation ofswitching element Q1 to the lowest normal frequency, OSC 12 maintainsthe switching frequency at the lowest normal frequency until theintermittent oscillation duration ends.

When FB voltage V_(FB) is equal to or lower than burst threshold voltageVth₂ at time t3, the output signal of comparator COMP3 is set to the Hlevel, which stops the switching operation of switching element Q1 andthen ends the intermittent oscillation duration. As described above, inthe lightly-loaded state with a relatively large load, OSC 12 switchesthe oscillation frequency to the first intermittent-operation frequency,the second intermittent-operation frequency, and the lowest normalfrequency in this order to perform the intermittent oscillatingoperation at the first intermittent-operation frequency, the secondintermittent-operation frequency, and the lowest normal frequency in theintermittent oscillation duration. In the lightly-loaded state with arelatively large load, a transition from the intermittent oscillatingoperation to the normal oscillating operation is smoothly performedbecause the switching frequency has been increased to and is maintainedat the lowest normal frequency in the intermittent oscillation durationas illustrated with a dotted line in FIG. 3.

FIG. 5 shows graphs illustrating FB voltage V_(FB), drain current ID ofswitching element Q1, and the oscillation frequency of OSC 12 in theintermittent oscillating operation with as small load as several mA atmicrocomputer standby, for example. In FIG. 5, a time axis isillustrated longer as it goes down. With as small load as several mA, FBvoltage V_(FB) becomes equal to or lower than burst threshold voltageVth₂ before time t1 is reached, in other words, before time T1 haspassed since the outputting of the oscillation-start detection signal,as illustrated in FIG. 5. Accordingly, the oscillation frequency doesnot switch to the second intermittent-operation frequency nor the lowestnormal frequency in the intermittent oscillation duration, and theintermittent oscillating operation is performed at the firstintermittent-operation frequency.

FIG. 6 shows graphs illustrating FB voltage V_(FB), drain current ID ofswitching element Q1, and the oscillation frequency of OSC 12 in theintermittent oscillating operation at several W control under alightly-loaded operating condition by setting and the like. In FIG. 6, atime axis is illustrated longer as it goes down. As illustrated in FIG.6, at the several W control under the lightly-loaded operatingcondition, FB voltage V_(FB) becomes equal to or lower than burstthreshold voltage Vth₂ before time t2 is reached, in other words, beforetime T2 has passed since the outputting of the first frequency switchingsignal. Accordingly, the oscillation frequency does not switch to thelowest normal frequency in the intermittent oscillation duration, andthe intermittent oscillating operation is performed at the firstintermittent-operation frequency and the second intermittent-operationfrequency.

As described above, since the intermittent oscillation duration changeswith a load, switching of the oscillation frequency of OSC 12 iscontrolled based on timing of a change in the first embodiment. Thisallows the intermittent oscillating operation with a reduced oscillationfrequency to have a wider load range. Performing the switching of theoscillation frequency in stages in accordance with an oscillation timeallows the intermittent oscillating operation including a low-frequencyoperation to have a sufficient load range. Although the first embodimentdescribes that the oscillation frequency is switched two times, thenumber of times to switch the oscillation frequency is not limited andmay be one, or three or more.

Second Embodiment

As illustrated in FIG. 7, controller IC 1 a according to a secondembodiment is provided with counter circuit 15 in place of timer circuit14 in the first embodiment.

Counter circuit 15 receives an output signal of comparator COMP3 and anoutput signal of OR circuit OR1. When a load is in a range to performthe intermittent oscillating operation, the output signal of comparatorCOMP3 is turned off by shifting from the H level to the L level, countercircuit 15 outputs an oscillation-start detection signal to OSC 12 andstarts counting of the number of oscillations based on the output signalof OR circuit OR1. Then, until the counted number of oscillationsreaches at a previously set first count number, counter circuit 15outputs, to OSC 12, a first frequency switching signal for setting theoscillation frequency to a first frequency lower than the lowest normalfrequency. When the counted number of oscillations reaches at the firstcount number, counter circuit 15 outputs, to OSC 12, a second frequencyswitching signal for setting the oscillation frequency to a secondfrequency higher than the first frequency. When the counted number ofoscillations reaches at a second count number larger than the firstcount number, counter circuit 15 may output, to OSC 12, the lowestnormal frequency switching signal for setting the oscillation frequencyto the lowest normal frequency. As described above, in the secondembodiment, switching of the oscillation frequency of OSC 12 iscontrolled based on the counted number of oscillations. The first countnumber and the second count number may be individually set asappropriate.

Third Embodiment

As illustrated in FIG. 8, controller IC 1 b according to a thirdembodiment is provided with voltage detection circuit 16 in place oftimer circuit 14 in the first embodiment. Voltage detection circuit 16receives an output signal of comparator COMP3 and IC power-supplyvoltage Vcc. When a load is in a range to perform the intermittentoscillating operation, the output signal of comparator COMP3 is turnedoff by shifting from the H level to the L level, and voltage detectioncircuit 16 outputs the oscillation-start detection signal to OSC 12 andstarts detection of IC power-supply voltage Vcc. Then, until ICpower-supply voltage Vcc reaches at a previously set first thresholdvoltage, voltage detection circuit 16 outputs, to OSC 12, a firstfrequency switching signal for setting the oscillation frequency of OSC12 to a first frequency lower than the lowest normal frequency. When ICpower-supply voltage Vcc reaches at a previously set second thresholdvoltage higher than the first threshold voltage, voltage detectioncircuit 16 outputs, to OSC 12, a second frequency switching signal forsetting the oscillation frequency to a second frequency higher than thefirst frequency. Increase in IC power-supply voltage Vcc depends on theintermittent oscillation duration. Thus, in the third embodiment,switching of the oscillation frequency of OSC 12 is controlled based ona detection result of IC power-supply voltage Vcc. A characteristic,such as the rate of increase, of IC power-supply voltage Vcc can beoptionally adjusted to some extent with smoothing capacitor C3 andregister R3. This allows the switching frequency of the intermittentoscillation duration to be kept at the first intermittent-operationfrequency, and switch timing to be set as appropriate. When ICpower-supply voltage Vcc is in a predetermined range, OSC 12 may changethe oscillation frequency in synchronization with IC power-supplyvoltage Vcc.

A frequency increase pattern of increasing the oscillation frequency maybe the oscillation frequency may be increased in accordance with apreviously set frequency increase pattern (in stages or linear) in theintermittent oscillation duration. In a normal oscillation mode, theoscillation frequency may be repeatedly changed in accordance with afrequency change pattern including oscillation frequencies that areaveraged to be the lowest normal frequency, in a duration (duration ofburst threshold voltage Vth₂ in FIG. 3<FB voltageV_(FB)<frequency-reduction end voltage Vb) in which the switchingoperation is performed at the lowest normal frequency. This arrangement,involving change of the oscillation frequency, is expected to beeffective for electromagnetic interference (EMI).

As described above, a switched-mode power supply device according to anembodiment performs the switching operation of switching element Q1connected to primary winding P of transformer T while applying, toprimary winding P of transformer T, direct-current voltage obtained byrectifying input voltage from a commercially-availablealternating-current source AC, so as to cause secondary winding S oftransformer T to induce a pulse voltage, which is then rectified andsmoothed by the secondary-side rectifying-smoothing circuit includingrectifying diode D1 and smoothing capacitor C2, to be output as anoutput voltage Vo to a load. The switched-mode power supply deviceincludes error amplifier 2, OSC 12 as an internal oscillation circuit(oscillator), and comparator COMP3 serving as an intermittentoscillation control circuit. Error amplifier 2 is configured to transmitthe error voltage to the primary side as a FB signal indicating thestate of the load, the error voltage being obtained by comparing outputvoltage Vo to the reference voltage. OSC 12 is configured to control theswitching frequency of the switching operation such that the switchingfrequency is reduced from the highest normal frequency to the lowestnormal frequency based on the FB signal in a load range from aheavily-loaded state to a lightly-loaded state in the normal oscillatingoperation that continuously performs the switching operation. ComparatorCOMP3 is configured to control the intermittent oscillating operationthat intermittently performs the switching operation stopping theswitching operation based on the FB signal when the load is lighter thana previously set reference. OSC 12 increases the switching frequencyfrom an intermittent-operation frequency (the firstintermittent-operation frequency) lower than the lowest normal frequencyin the intermittent oscillation duration that performs the switchingoperation in the intermittent oscillating operation.

This configuration automatically controls the intermittent oscillationduration, the switching frequency, and the number of oscillations basedon the FB signal, thereby having a high versatility and achieving animproved efficiency in the intermittent oscillating operation. Forexample, when the load is small, the intermittent oscillating operationis performed at the intermittent-operation frequency lower than thelowest normal frequency, which leads to an improved efficiency and areduced oscillation frequency and achieves reduction of switch loss thatloss is dominant. In order to prevent a transition from the intermittentoscillating operation to a steady oscillating operation from occurringearlier than expected when the switching frequency of the intermittentoscillation duration is reduced, burst threshold voltage Vth₂ for atransition to the intermittent oscillating operation needs to be sethigh. Setting burst threshold voltage Vth₂ to be high causes a largedrain current ID of the intermittent oscillation duration, which affectssound noise and output ripple of the transformer, for example. In thepresent embodiment, in the intermittent oscillation duration, theswitching frequency is increased from the intermittent-operationfrequency (first intermittent-operation frequency), and thus burstthreshold voltage Vth₂ does not need to be set high, and drain currentID in the intermittent oscillation duration can be reduced.Specifically, when burst threshold voltage Vth₂ is fixed, a lowerswitching frequency results in a lower output supply electric power, anda higher switching frequency results in a higher output supply electricpower. Thus, in the intermittent oscillation duration, the increase fromthe first intermittent-operation frequency leads to a gradual increasein the output supply electric power, thereby reducing the sound noiseand output ripple of the transformer, for example.

According to another embodiment, different frequencies (the firstintermittent-operation frequency and the second intermittent-operationfrequency) are set as the intermittent-operation frequency, and OSC 12increases the switching frequency by switching from the firstintermittent-operation frequency, which is a lowest frequency, to thesecond intermittent-operation frequency higher than the firstintermittent-operation frequency in the intermittent oscillationduration.

According to another embodiment, OSC 12 increases the switchingfrequency from the intermittent-operation frequency toward the lowestnormal frequency and maintains the switching frequency when theswitching frequency reaches the lowest normal frequency in theintermittent oscillation duration.

This configuration enables a transition from the intermittentoscillating operation to the normal oscillating operation to be smoothlyperformed because the switching frequency has been increased to and ismaintained at the lowest normal frequency in the intermittentoscillation duration in the lightly-loaded state with a relatively largeload.

According to another embodiment, OSC 12 may set the switching frequencyback to the intermittent-operation frequency before starting theswitching operation at each intermittent oscillation duration.

According to another embodiment, the switched-mode power supply devicefurther includes timer circuit 14 configured to measure a time fromstart of the intermittent oscillation duration. OSC 12 increases theswitching frequency from the intermittent-operation frequency based onthe time measured by timer circuit 14 at each intermittent oscillationduration.

According to another embodiment, the switched-mode power supply devicefurther includes counter circuit 15 configured to count the number ofoscillations from start of the intermittent oscillation duration. OSC 12may increase the switching frequency from the intermittent-operationfrequency based on the number of oscillations counted by counter circuit15 at each intermittent oscillation duration.

According to another embodiment, the switched-mode power supply devicefurther includes voltage detection circuit 16 configured to detect ICpower-supply voltage Vcc obtained by rectifying and smoothing a pulsevoltage generated across auxiliary winding D of transformer T. OSC 12may increase the switching frequency from the intermittent-operationfrequency based on a detection result of IC power-supply voltage Vcc byvoltage detection circuit 16 at each intermittent oscillation duration.

Conventional techniques such as disclosed in Patent literature 1 definethe minimum number of oscillations, which reduces its versatility forsupporting various power-supply specifications, and sometimes suppliesan amount of energy more than needed, which results in a large outputripple and thus a reduced efficiency of an intermittent oscillatingoperation.

The embodiments described above solve this problem of the conventionaltechnique, and provide a switched-mode power supply device having a highversatility and achieving an improved efficiency in an intermittentoscillating operation.

The embodiments automatically control an intermittent oscillationduration, a switching frequency, and a number of oscillations based on aFB signal, thereby having a high versatility and achieving an improvedefficiency in an intermittent oscillating operation.

The invention includes other embodiments in addition to theabove-described embodiments without departing from the spirit of theinvention. The embodiments are to be considered in all respects asillustrative, and not restrictive. The scope of the invention isindicated by the appended claims rather than by the foregoingdescription. Hence, all configurations including the meaning and rangewithin equivalent arrangements of the claims are intended to be embracedin the invention.

What is claimed is:
 1. A switched-mode power supply device, comprising:a transformer comprising a primary winding in a primary side to which aDC voltage is applied and a secondary winding; a switching circuitconnected to the primary winding; a rectifying-smoothening circuit thatrectifies and smoothens a pulse voltage generated at the secondarywinding, by switching the switching element; an output unit that outputsan output voltage obtained by the rectifying-smoothening circuit to aload; an error amplifier that outputs an error voltage to the primaryside as a feedback signal indicating a state of the load, the errorvoltage being obtained by comparing the output voltage to a referencevoltage; an oscillator that controls a switching frequency of theswitching operation such that the switching frequency is reduced from ahighest normal frequency to a lowest normal frequency based on thefeedback signal in a load range from a heavily-loaded state to alightly-loaded state in a normal oscillating operation that continuouslyperforms the switching operation; and an intermittent oscillationcontrol circuit that intermittently performs the switching operation bystopping the switching operation based on the feedback signal when theload is lighter than a previously set reference, wherein the oscillatorincreases the switching frequency from an intermittent-operationfrequency lower than the lowest normal frequency in an intermittentoscillation duration in which the switching operation is performed inthe intermittent oscillating operation.
 2. The switched-mode powersupply device according to claim 1, wherein different frequencies areset as the intermittent-operation frequency, and the oscillatorincreases the switching frequency by switching theintermittent-operation frequency from a low frequency to a highfrequency in the intermittent oscillation duration.
 3. The switched-modepower supply device according to claim 1, wherein the oscillatorincreases the switching frequency from the intermittent-operationfrequency toward the lowest normal frequency and maintains the switchingfrequency when the switching frequency reaches the lowest normalfrequency in the intermittent oscillation duration.
 4. The switched-modepower supply device according to claim 2, wherein the oscillatorincreases the switching frequency from the intermittent-operationfrequency toward the lowest normal frequency and maintains the switchingfrequency when the switching frequency reaches the lowest normalfrequency in the intermittent oscillation duration.
 5. The switched-modepower supply device according to claim 1, wherein the oscillator setsthe switching frequency back to the intermittent-operation frequencybefore starting the switching operation at each intermittent oscillationduration.
 6. The switched-mode power supply device according to claim 2,wherein the oscillator sets the switching frequency back to theintermittent-operation frequency before starting the switching operationat each intermittent oscillation duration.
 7. The switched-mode powersupply device according claim 1, further comprising: a timer circuitthat measures a time from start of the intermittent oscillationduration, wherein the oscillator increases the switching frequency fromthe intermittent-operation frequency based on the time measured by thetimer circuit at each intermittent oscillation duration.
 8. Theswitched-mode power supply device according claim 2, further comprising:a timer circuit that measures a time from start of the intermittentoscillation duration, wherein the oscillator increases the switchingfrequency from the intermittent-operation frequency based on the timemeasured by the timer circuit at each intermittent oscillation duration.9. The switched-mode power supply device according to claim 1, furthercomprising: a counter circuit that counts the number of oscillationsfrom start of the intermittent oscillation duration, wherein theoscillator increases the switching frequency from theintermittent-operation frequency based on the number of oscillationscounted by the counter circuit at each intermittent oscillationduration.
 10. The switched-mode power supply device according to claim2, further comprising: a counter circuit that counts the number ofoscillations from start of the intermittent oscillation duration,wherein the oscillator increases the switching frequency from theintermittent-operation frequency based on the number of oscillationscounted by the counter circuit at each intermittent oscillationduration.
 11. The switched-mode power supply device according to claim1, further comprising: a voltage detection circuit that detects apower-supply voltage obtained by rectifying and smoothing a pulsevoltage generated across an auxiliary winding of the transformer,wherein the oscillator increases the switching frequency from theintermittent-operation frequency based on a detection result of apower-supply voltage by the voltage detection circuit at eachintermittent oscillation duration.
 12. The switched-mode power supplydevice according to claim 2, further comprising: a voltage detectioncircuit that detects a power-supply voltage obtained by rectifying andsmoothing a pulse voltage generated across an auxiliary winding of thetransformer, wherein the oscillator increases the switching frequencyfrom the intermittent-operation frequency based on a detection result ofa power-supply voltage by the voltage detection circuit at eachintermittent oscillation duration.